The present invention relates to a silicon germanium (SiGe) crystal preferably used as a thermoelectric element material and a thermoelectric element using the same.
When a P-type semiconductor material and a N-type semiconductor material are joined each other at two junctions and a temperature difference is given between the two junctions, there is generated thermoelectromotive force between them by the so-called Seebeck effect.
A thermoelectric element employing the above principle has no movable part and is simple in construction and hence with this element an energy direct conversion system which is high in reliability, long in lifetime and easy in maintenance may be quite within the bounds of possibility. Therefore, various kinds of thermoelectric element materials have conventionally produced and developed.
Among them, SiGe has been known as a typical thermoelectric element material of chemical stability and many proposals on improvements of its performance and production processes have been hitherto presented [for example, Japanese Patent Laid-open Publication No. 61-149453 (U.S. Pat. No. 4,711,971, European Patent No. 185499), Japanese Patent Laid-open Publication No. 8-56020, Japanese Patent No. 2623172].
The performance index Z which is an index of performance of the thermoelectric element is given by the following equation (1):
Z=xcex12"sgr"/Kxe2x80x83xe2x80x83(1)
[In the equation (1), xcex1 is the Seebeck coefficient, "sgr" the electric conductivity, and K the thermal conductivity.]
The performance indexes Z of various thrmoelectric element materials are shown in FIG. 7 in relation with the temperature. As is apparent from FIG. 7, in the case of an SiGe polycrystal obtained by a conventional production process, the performance index thereof is inferior to that of for example a tellurium based thermoelectric material such as Bi2Te2 or PbTe at a practical temperature ranging from 200xc2x0 C. especially to 600xc2x0 C., which is a weak point thereof in practical use.
Under these circumstances, in order to improve the performance index Z by raising the electric conductivity of the material with increase of concentrations of conduction electrons and holes thereof, there have been performed attempts wherein as dopants Group III elements such as B, Al and Ga are added into the P-type material, Group V elements such as P, As and Sb into the N-type material, and as disclosed in Japanese Patent Laid-open Publication Nos. 61-14953 and 8-56020, metals such as Pb, Sn, Fe, Ni and Cr, and silicides thereof into the material.
With these improvements, the performance index Z of SiGe has been improved, but there is a great demand for further improvements of the performance index for practical use.
There has been another problem, since an ingot of SiGe is produced by means of the casting method or the Bridgemann process in which components Si, Ge and additives such as dopants are mixed to form a mixture in respective predetermined amounts, and then molten to become a composition as homogeneous as possible, followed by cooling, or by means of powder sintering the mixture, the obtained ingot is collective concretion of crystal grains.
For the above reason, there have arisen the following obstacles {circumflex over (1)}xcx9c{circumflex over (3)} that hinders real practical use of SiGe as the thermoelectric material: {circumflex over (1)} As scattering of carriers at grain boundaries of the ingot cannot be avoided, the improvement of the electric conductivity thereof is impeded. {circumflex over (2)} As grain boundary segregation arises at a practical temperature of 200xc2x0 C. or higher, especially in the vicinity of a high temperature heat source of 500xc2x0 C. or higher, the characteristics thereof degrade in process of time. {circumflex over (3)} Local inhomogeneity of the composition in the ingot is inevitable, thereby characteristics being further degraded and cracking easily occurring not only in mechanical processing but during use.
Through a diligent research conducted with a view to the drawbacks of the polycrystalline SiGe obtained by the conventional production process, the present inventors have acquired an inventive idea that the drawbacks are solved and an SiGe thermoelectric element practically usable can be realized by increasing sizes of crystal grains forming an SiGe block, preferably of a single crystal. As a result of various actual investigations of the idea, the present inventors have succeeded in producing an SiGe crystal ingot composed of crystal grains of 5xc3x9710xe2x88x925 mm3 or more in size almost over all the range of x of SixGe1xe2x88x92x (0 less than x less than 1) by means of the Czochralski method.
It is an object of the present invention to provide an SiGe crystal material that realizes the improvement of the performance index as the thermoelectric element and is excellent in machinability, there arising neither degradation in the characteristics nor cracking during use.
To solve the above problem, an SixGe1xe2x88x92x (0 less than x less than 1) crystal of the present invention is characterized in that crystal grains forming said crystal are 5xc3x9710xe2x88x925 mm3 or more in size.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal is preferably produced by means of a pulling method.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal has preferably an absolute value of the Seebeck coefficient in the range of 100 to 700 xcexcV/K.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal has preferably a value of a thermal conductivity in the range of 1 to 20 W/mxc2x7K.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal has preferably a value of the electric conductivity in the range of 101 to 105 W/xcexa9xc2x7m.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal has more preferably an absolute value of the Seebeck coefficient in the range of 100 to 700 xcexcV/K, a value of the thermal conductivity in the range of 1 to 20 W/mxc2x7K and a value of the electric conductivity in the range of 101 to 105 W/xcexa9xc2x7m.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal has preferably a value of x in the range of 0.6 to 0.8.
A P-type thermoelectric material may be produced by adding an element selected from B, Al or Ga into the SixGe1xe2x88x92x (0 less than x less than 1) crystal.
An N-type thermoelectric material may be produced by adding an element selected from P, As or Sb into the SixGe1xe2x88x92x (0 less than x less than 1) crystal.
The SixGe1xe2x88x92x (0 less than x less than 1) crystal is preferably a single crystal.
A thermoelectric element of the present invention is characterized by using the SixGe1xe2x88x92x (0 less than x less than 1) crystal described above.
When sizes of crystal grains forming an SixGe1xe2x88x92x (0 less than x less than 1) crystal are increased, the mechanical strength of the crystal is improved and sustained highly especially at a high temperature at which a thermoelectric element is used, so that the crystal is mechanically stable under working environment of the element, and hence degradation of the element may be suppressed. These functions are more conveniently enhanced in the case where the crystal ingot is a single crystal.